1. Field of The Invention
This invention relates to amplifiers and, more particularly, to an adaptive power amplifier architecture.
2. Description of Related Art
An ideal power amplifier amplifies an input signal with no waveshape alteration. The ideal power amplifier is therefore characterized as having a transfer function (input signal vs. output signal) which is linear with no transfer function discontinuities. In practice, a power amplifier, however, has a transfer function with nonlinear and xe2x80x9clinearxe2x80x9d regions. Whether the power amplifier is operating in a linear or nonlinear region depends on the amplitude of the input signal. For the power amplifier to achieve as near to linear operation as possible, the power amplifier is designed to operate within its linear region given the range of possible input signal amplitudes. If the input signal has an amplitude which causes the power amplifier to operate outside the linear region, the power amplifier introduces nonlinear components or distortion to the signal. When the input signal possesses peak amplitudes which cause the amplifier to compress, to saturate (no appreciable increase in output amplitude with an increase in input amplitude) or to shut-off (no appreciable decrease in output amplitude with a decrease in input amplitude), the amplifer is being overdriven, and the output signal is clipped or distorted in a nonlinear fashion. In addition to distorting the signal, the clipping or nonlinear distortion of the input signal, generates spectral regrowth or adjacent channel power (ACP) that can interfere with an adjacent frequency.
In wireless communications systems, high power amplification of signals for transmission are commonly encountered with very large peak to average power ratios. (PAR). For example, in a time division multiple access (TDMA) system, when multiple carriers signals are combined for amplification with a power amplifier, the resulting PAR is about 9 dB for a large number of carriers. In a code division multiple access (CDMA) system, a single loaded 1.25 Mhz wide carrier can have a PAR of 11.3 dB. These signals have to be amplified fairly linearly to avoid generating ACP. To satisfy the linearity requirement, power amplifiers are usually operated in Class A and Class AB configurations. To be able to handle large signal peaks linearly, the amplifiers are biased at high bias currents. The efficiency of the amplifiers is low because of the high bias and the high peak to average power ratios.
Accordingly, efficiency of the amplifier is inversely related to the ability to handle high peaks in a linear fashion. To achieve a high degree of linearity, the amplifiers are biased to operate, in class A or xe2x80x9cslightxe2x80x9d class AB (meaning class AB operation that is closer to class A than to class B). Maximum theoretical AC to DC efficiency achievable for class A operation is 50%, whereas that of a class AB amplifier is between 50 and 78.5% (the latter representing the maximum efficiency of a class B amplifier). The closer the particular class AB operation is to class A, the lower the maximum efficiency. For amplifiers employing field effect transistors, the class of operation is set in accordance with the gate voltage applied, which controls the quiescent (idle) drain current. For class A operation, the gate voltage is set so that the idle drain current is approximately in the middle of the range between pinch-off and saturation. Class B amplifiers are biased near pinch-off, resulting in a rectified drain current waveform. Class AB amplifiers are biased in between the bias points of classes A and B.
Typically, strict linearity requirements in modern wireless communication systems dictate the use of the relatively inefficient class A or slight class AB modes. As a result, significant DC power is dissipated by the amplifiers, thereby generating heat which must be controlled to avoid degrading amplifier performance and reliability. Hence, the use of elaborate heat sinks and fans become a necessary by-product of the high linearity system. Naturally, these measures add to the cost, size and weight of the base station equipment. As the number of wireless communications users continues to grow, so do the number of base stations and the need to keep them small, light and inexpensive. Thus, a great deal of research has focused on the quest to improve amplifier efficiency in these and other systems.
Various methods are used to enable the use of more cost-effective and more power efficient amplifiers while maintaining an acceptable level of linearity. Feed-forward correction is routinely deployed in modern amplifiers to improve the linearity of the main amplifier with various input patterns. The essence of the feed forward correction is to isolate the distortion generated by the main amplifier on a feed forward path. The distortion is provided to a correction amplifier on the feed forward path which amplifies the distortion. The distortion on the feed forward path is combined with the distortion on the main signal path to cancel the distortion on the main signal path. Pre-distortion techniques distort the input signal prior to amplification by taking into account the transfer function characteristics for the amplifier. As such, the desired amplified signal is achieved from the pre-distorted input signal. A technique-described by Adel A. M. Saleh and Donald C. Cox, xe2x80x9cImproving the Power-Added Efficiency of FET Amplifiers Operating with Varying Envelope Signals,xe2x80x9d IEEE Transactions On Microwave Theory and Techniques, Vol. 31, No. 1, January 1983 uses the input signal envelope to dynamically adjust the amplifier bias such that a high bias is only applied when a large peak is encountered. Baseband processing techniques can also be used to improve the efficiency and/or linearity of the power amplifier architecture.
Wireless base stations use a variety of radio frequency (RF) amplifers in both single carrier and multicarrier configurations operating in Class A and Class AB modes. FIG. 1 shows a typical feed-forward power amplifier architecture 10 which includes a main amplifier 12 to amplify the input signal on the main signal path 13 and a correction amplifier 14 used in reducing the distortion produced from the main amplifier 12. In performing feed forward correction, the distortion generated by the amplifier 12 is isolated on a distortion cancellation path 16. To isolate the distortion on the distortion cancellation or feed forward path 16, a coupler 17 provides a version of the amplified input signal and distortion from the output of the main amplifier 12 onto a coupling path 18 to a coupler 19. A coupler 20 provides a version of the input signal from the main signal path 13 to the coupler 19. The coupler 19 combines the amplified input signal and distortion from the coupling path with the version of the input signal from the feed forward path 16. As such, the input signals cancel and the distortion is left on the distortion cancellation path 16. The distortion is provided to the correction amplifier 14 which amplifies the distortion on the distortion cancellation path 16. A combiner 22 cancels the distortion on the main signal path 13 with the distortion on the distortion cancellation path 16 to reduce the distortion produced from the main amplifier 12.
Other power amplifier architectures are possible which use different structures and do not use a correction amplifier 14 as described above to reduce the nonlinear distortion of the signal. For example, the correction amplifier 14 can be replaced with a second amplifier or amplifiers in an arrangement where the amplifiers amplify versions of the original signal, and the amplified versions of the original signal are combined to produce the amplified signal while producing reduced distortion. For example, U.S. Pat. No. 5,917,375 issued on Jun. 29, 1999 entitled xe2x80x9cLow Distortion Amplifier Circuit with Improved Output Powerxe2x80x9d describes a power amplification architecture using multiple amplifiers.
In the embodiment of FIG. 1, the main amplifier 12 is configured as parallel amplifier stages of like amplifiers to provide the same gain as the individual amplifiers while increasing the output; power of the main amplifier 12 with each amplifier stage 26a-d. The main amplifier 12 includes an arrangement of 1:2 splitters 24a-cwhich split the input signal among parallel amplifiers 26a-d. An arrangement of 2:1 combiners 28a-c combines the outputs of the parallel amplifiers 26a-d to produce an amplified signal on the main signal path 13. The main amplifier 12 has the same gain as an individual amplifier 26a-n but the power handling capability of the main amplifier 12 is increased by the power handling capability of each individual amplifier 26a-d. 
In the described feed forward architecture, the main amplifier 12 is the largest single contributor to the overall power consumption in CDMA, TDMA and frequency division multiple access (FDMA) base stations. Due to the potential for high peak powers, the main amplifier 12 is biased with a high current to be able to handle those peak powers when they do occur. As such, the efficiency of the main amplifier 12 is typically less than 30%. This low efficiency leads to higher power consumption, shorter battery backup time, lower overall reliability and higher operating temperatures. Accordingly, there is a need for a more efficient power amplifier architecture.
The present invention involves a power amplifier system using adaptive distribution of signals through an amplifier architecture of parallel amplifier stages. For example, the power amplifier system can adjust the operation of individual amplifier stages in the amplifier arrangement, for example to reduce power handling capability of the amplifier stages based on information of the signal to be amplified by the amplifier arrangement, such as the power level. To take advantage of the adjusted operation of the amplifier stages, the power amplifier system uses a variable power divider to reapportion the input-signal power among the amplifier stages and at least one variable combiner to adaptively combine amplified signals from, the amplifier stages in changing proportions. As such, the power amplifier system is reconfigurable and/or can adapt to changing conditions to provide improved performance and/or efficiency.